In prior art processes for formation of volume-phase holograms, data is stored as holograms resulting from the interference of signal and reference beams within a holographic recording medium comprising a homogenous mixture of at least one polymerizable monomer or oligomer and a polymeric binder; the polymerizable monomer or oligomer must of course be sensitive or sensitized to the radiation used to form the interference fringes. In the illuminated regions of the interference pattern, the monomer or oligomer undergoes polymerization to form a polymer that has a refractive index different from that of the binder. Diffusion of the monomer or oligomer into the illuminated regions, with consequent chemical segregation of binder from these areas and its concentration in the non-illuminated regions, produces spatial separation between the polymer formed from the monomer or oligomer and the binder, thereby providing the refractive index modulation needed to form a hologram. Typically, after the holographic exposure, a post-imaging blanket exposure of the medium to actinic radiation is required to complete the polymerization of the monomer or oligomer and fix the hologram. When holograms are multiplexed co-locationally, such as by multiple holographic exposures at different angle conditions, a post-imaging blanket exposure of the medium to actinic radiation may also be required to complete the polymerization of the monomer or oligomer and fix the multiplexed holograms.
One important potential use of volume holograms is in digital data storage; the three dimensional nature of a volume hologram, which refers to the storage of each bit as a hologram extending throughout the entire volume of the recording medium, renders volume holograms suitable for use in high capacity digital data storage. A group of bits can be encoded and decoded together as a two dimensional array of bits referred to as a page. Various multiplexing methods, such as angular, spatioangular, shift, wavelength, phase-code, and related methods, are used to store multiple pages co-locationally within the same volume or in partially overlapping volumes.
Microholograms, or bit-wise volume holograms, can be recorded independently in layers, such that each layer comprises a 2-dimensional array of bit-wise locations and the number of layers in the depth direction is related to the thickness of a layer relative to the overall thickness of the recording medium. Eichler et al. in IEEE, 4, 5, 840 (1998) describe a method for recording microscopic Bragg-reflectors, with a radius of 1.8 μm and 12 μm depth, in Dupont Omnidex HRF-800 photopolymer films with 20 micron thickness using lenses of a DVD pickup. The number of layers was limited to two by the thickness of the photopolymer recording medium and the beam waist used. In the z direction, or parallel to the counterpropagating direction of the writing beams, there is a periodic modulation with a strongly profiled envelope of the grating fringes, whereas in the orthogonal directions, x and y, the microholograms are characterized by a Gaussian refractive index modulation. Use of this method, however, for recording many such layers in thicker currently available photosensitized photopolymer recording media requires knowledge of the recording sensitivity of a region in the desired layer at the time of recording a particular microhologram, as the local recording sensitivity is affected by recording of previous microholograms in nearby regions of other layers and even by reading of data stored as holograms. A disk containing 10 or more such layers would be an attractive alternative to multilayer CD and/or DVD technology, as the density of data per layer would be similar to that obtained with CD and/or DVD disks and the overall area data density would scale linearly with thickness or number of layers. Strickler et al. in U.S. Pat. No. 5,289,407 describe a non-holographic, multi-layered, refractive index-perturbation optical storage system. Bits are stored at the high intensity focus of a single laser beam as localized perturbations in the index of refraction of a photopolymer.
Photopolymerizable holographic recording media for write-once-read-many (WORM) applications should ideally exhibit lengthy pre-recording shelf life and good recording sensitivity. This particular performance characteristic, however, remains as one of the most difficult to achieve for photopolymerizable holographic recording media that are designed for data storage applications. Typically initiator species form over time, as a result of thermal decomposition processes that take place at room temperature, and thus unintended polymerization occurs. A consequence of short prerecording shelf life is a monotonic and significant decline in recording sensitivity as exhibited by photopolymerizable recording media. The use of conventional photopolymerizable recording media for data storage therefore requires continual knowledge of recording sensitivity as a function of pre-recording time. Moreover, sensitivity may be different for one location on a disk versus another depending upon whether nearby locations were imaged at earlier times, and to what degree polymerization reactions persisted into the surrounding regions. Additionally, if recording has been carried out at a particular location, but in a way such that the entire recording ability of the material at that location has not been fully realized, then subsequent recording of data could be effected at this location by use of various multiplexing methods. This, of course, is only practical when the recording sensitivity is known at the onset of each recording event. The sensitivity in such cases depends on both the time of storage of the material and the shelf life characteristics of the material, as well as upon the extent of photopolymerization that has taken place during previous recording events at a location.
A photopolymerizable holographic recording medium comprising an initiator that creates the requirement of a threshold event for initiation of additional polymerization would be an attractive candidate for WORM data recording applications. The term “threshold” is used herein to refer to a nonlinear relationship between the intensity or energy delivered to the data storage medium and the number of initiated polymerization events that occur. The polymerization process that takes place subsequent to and as a result of the threshold event amplifies the effect of the threshold initiation process. A threshold event for recording of data is a particularly attractive performance characteristic when this event results in local variation of the refractive index modulation in a controlled manner, and if the energy requirements for the event are reasonable in comparison to intensities available from common laser sources. For example, in the case of microscopic Bragg-reflectors, known as microholograms [see for example Eichler et al. in SPIE Vol. 3109, 239 (1997) and in IEEE, 4, 5, 840 (1998)], the term local would refer to the focal region of light transmitted through an imaging lens, and which is defined by a certain radius and depth in the material. A localized non-holographic alteration of the index of refraction, such as by polymerization within the focal region of an imaging lens, alters the refractive index modulation of a hologram that exists in the same region. Hesselink et al. in PCT application WO 99/39248 describe a multi-depth, bit-wise optical data storage system whereby a format hologram is first stored in a holographic storage medium, and data are then stored as selective, microlocalized alterations of the format hologram.